The present invention relates to an apparatus for driving typically a liquid-crystal display device, a method adopted by the apparatus and a display apparatus using the liquid-crystal display device.
In recent years, due to its thinness and lightness, a liquid-crystal display device is widely used as a device for displaying an image.
As shown in FIG. 10, a liquid-crystal display device 100 includes a driving substrate 102, on which pixel electrodes 101 are provided to form a 2-dimensional layout, a facing substrate 104 having a facing electrode 103 and a liquid crystal 105. The driving substrate 102 and the facing substrate 104 are attached to each other, sandwiching a gap with a predetermined thickness allowing the electrodes 101 and 103 to face each other. The liquid crystal 105 is sealed inside the gap.
The orientation of the liquid crystal 105 varies in accordance with the strength of an electric field applied to the liquid crystal 105. That is to say, the transmittivity of the liquid crystal 105 changes in accordance with the magnitude of a voltage applied between the pixel electrodes 101 and the facing electrode 103. Thus, if a voltage of an image signal is applied to a pixel electrode 101 in the liquid-crystal display device 100, the transmittivity of a portion corresponding to the pixel electrode 101 changes. As a result, if a light beam such as a backlight ray is radiated to the picture electrodes 101, an image represented by the image signal can be displayed.
As described above, the orientation of the liquid crystal 105 varies in accordance with the strength of an electric field applied to the liquid crystal 105. As shown in FIG. 11, however, the transmittivity does not change even if the polarity of the applied electric field is reversed. That is to say, the transmittivity for an electrical-potential difference of +V1 applied between the pixel electrodes 101 and the facing electrode 103 is equal to the transmittivity for an electrical-potential difference of −V1 applied between the pixel electrodes 101 and the facing electrode 103. It is to be noted that FIG. 11 is a diagram showing the transmittivity of a liquid crystal in the so-called normally black mode. A liquid crystal in the so-called normally black mode is a liquid crystal having a transmittivity of 0 for an applied electrical field of 0. However, the relation between the transmittivity and the strength of the applied electrical field shown in the figure also holds true of a liquid crystal in the so-called normally white mode. That is to say, the transmittivity does not change even if the polarity of the applied electric field is reversed. A liquid crystal in the so-called normally white mode is a liquid crystal having a maximum transmittivity for an applied electrical field of 0.
In addition, a moving-image signal is displayed in the liquid-crystal display device 100 by normally carrying out frame hold driving in which, for one frame, a voltage is applied to the pixel electrodes 101 once and the applied voltage is held continuously till the next frame is displayed to sustain the display of the current frame. In the case of the frame hold driving, however, moving-image senility is felt in the visual-sense optic of a human due to a residual image feeling. As one of methods for avoiding the senility caused by the residual-image feeling, the speed to switch an image is raised.
As one of liquid-crystal-driving methods of raising the speed to switch an image from one frame to another, there is a method whereby frames of an input moving-image signal representing an original moving image are interpolated to generate a moving-image signal having a shorter frame period as shown in FIG. 12 and the moving-image signal having a shorter frame period is displayed in a liquid-crystal display device. The moving-image signal having a shorter frame period is a signal representing a moving image after frame interpolations. By shortening the frame period, the speed to renew an image displayed on a liquid-crystal display device can be increased.
In addition, if ion biasing occurs in the liquid crystal 105, a burn-in phenomenon is observed as is commonly known. In this burn-in phenomenon, a characteristic representing a relation between a voltage and a gradation can no longer be reproduced. If the worst comes to the worst, the burn-in phenomenon results in disassembly of the material.
In order to solve the above problem, the conventional liquid-crystal display device adopts a driving method whereby the voltage applied to the liquid crystal is inverted from a positive polarity to a negative one and vice versa periodically every image-switching period, that is, every frame (or every field). For more information on this driving method, refer to a document such Japanese Patent Laid-open No. Hei 4-299387. As shown in FIG. 13, in the liquid-crystal display device 100, the polarity of a signal voltage Vsin applied to each of the pixel electrodes 101 is inverted every frame period with a common voltage Vcom taken as the center of inversion. In this case, the common voltage Vcom is a voltage applied to the facing electrode 103.